Method for manufacturing optical disc master and method for manufacturing optical disc

ABSTRACT

An apparatus for manufacturing an optical disc master, having (a) a turntable upon which is received a disc having a resist layer composed of a resist material, the resist material comprising an incomplete oxide of a transition metal on a substrate, the oxygen content of the incomplete oxide being smaller than the oxygen content of the stoichiometric composition corresponding to a valence of the transition metal, which increases the absorption of ultraviolet rays and visible light rays, the resist material being an amorphous inorganic material containing an oxide; and (b) an exposure system operatively configured to selectively expose the resist layer to ultraviolet rays or visible light.

RELATED APPLICATION DATA

This application is a continuation of application Ser. No. 12/635,314,filed Dec. 10, 2009, which is a continuation of application Ser. No. No.10/498,044, filed, Jun. 8, 2004, which is the United States Nationalstage of PCT/JP2003/12236, filed Sep. 25, 2003, all of which areincorporated herein in their entireties to the extent permitted by law.Priority is claimed to Japanese Patent Application JP 2002-297892, filedin the Japanese Patent office on Oct. 10, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to a method for manufacturing a highlyaccurate optical disc master, and to a method for manufacturing anoptical disc produced by using the master.

Recently, recording media that record and store a wide variety ofinformation have been remarkably developing. In particular, regarding acompact recording medium, as the recording system is changed from amagnetic recording medium to an optical recording medium, the recordingcapacity has been increasing from an order of mega bytes (MB) to anorder of gigabytes (GB).

The optical recording medium has changed from Compact Disc™ (CD) to anoptical disc in recent years. A read-only optical disc, i.e., a digitalversatile disc read-only memory (DVD-ROM), being 12 cm in diameter hasan information capacity of 4.7 GB on the single side. This disc canrecord images that correspond to the recording for two hours in NationalTelevision System Committee (NTSC) color television system.

However, as information and communication technology and imageprocessing technology have rapidly developed in recent years, even theabove optical disc requires a several fold recording capacity relativeto the present capacity. For example, a next-generation optical disc,which is an extension of a digital video disc being 12 cm in diameter,requires an information capacity of 25 GB on the single side. This disccan record images that correspond to the recording for two hours in thedigital high vision system.

The optical disc is composed of an optically clear substrate, forexample, polycarbonate. Fine irregular patterns such as pits and groovesthat represent information signals are formed on one main surface on thesubstrate. A reflecting film, i.e., a metal thin film composed of, forexample, aluminum, is formed on the fine irregular patterns.Furthermore, a protective film is formed on the reflecting film.

In the above recording medium, minimizing the irregular pattern canincrease the recording density, and consequently, can increase therecording capacity. A process for manufacturing an optical disc, whichrelates to the minimizing of the irregular pattern on the optical disc,will now be described with reference to FIG. 10.

A resist layer 91 is uniformly formed on a substrate 90 (FIG. 10( a)).

Subsequently, the resist layer 91 is selectively exposed according to asignal pattern (FIG. 10( b)). The resist layer 91 is developed toproduce a master 92 having a predetermined irregular pattern thereon(FIG. 10( c)). An example of the known method for producing this masterwill now be described.

A glass substrate having a sufficiently smooth surface is used as thesubstrate. The substrate is disposed on a rotatable table. While theglass substrate is rotated at a predetermined speed, a photosensitiveresist, i.e., photo resist (organic resist) is applied on the substrate.The glass substrate is further rotated in order to spread the photoresist. Thus, the resist layer is formed on the whole area by spincoating. Subsequently, the photo resist is exposed with recording lasersuch that the photo resist has a predetermined pattern. Thus, a latentimage corresponding to information signals is formed on the substrate.Then, the substrate is developed with a developer to remove the exposedareas or the unexposed areas of the photo resist. In this way, a resistmaster is produced. The resist master 92 includes the glass substrateand the photo resist layer formed thereon and having the predeterminedirregular pattern.

Then, a metallic nickel film is formed on the irregular pattern of theresist master 92 by electroforming (FIG. 10( d)). The nickel film islifted off from the resist master 92. Subsequently, a predeterminedprocess is performed to produce a molding stamper 93 having theirregular pattern of the resist master 92 (FIG. 10( e)) .

Polycarbonate, which is a thermoplastic resin, is molded by injectionmolding using the molding stamper 93 to form a resin disc substrate 94(FIG. 10( f)) . The stamper is removed (FIG. 10( g)) , and then areflecting film 95 composed of an aluminum alloy (FIG. 10( h)) and aprotective film 96 are formed on the irregular surface of the resin discsubstrate 94 to produce an optical disc (FIG. 10( i)).

As described above, in order to produce the fine irregular pattern onthe optical disc, the pattern is reproduced on the substrate accuratelyand quickly by the use of the stamper on which the fine irregularpattern is formed with high precision. In terms of the precedentprocess, the precision of the fine irregular pattern on the optical discdepends on the cutting process, i.e., the process in which the resistlayer is exposed with laser to form the latent image.

For example, according to the above read-only DVD (DVD-ROM) having theinformation capacity of 4.7 GB, cut portions are formed on the stampersuch that a pit line (0.4 μm in the minimum pit length, 0.74 μm in thetrack pitch) is formed in a spiral shape. In order to form the cutportions, laser having the wavelength of 413 nm and an objective lenshaving the numerical aperture NA of about 0.90 (for example 0.95) areused.

The minimum pit length P (μm) to be exposed is represented by followingFormula (1):

P=K·λ/NA  (1)

wherein λ (μm) represents a wavelength of the light source, NArepresents a numerical aperture of the objective lens, and K representsa proportionality constant.

The wavelength λ of the light source and the numerical aperture NA ofthe objective lens depend on the specification of laser equipment, andthe proportionality constant K depends on the combination of the laserequipment and the resist master.

When the optical disc having the information capacity of 4.7 GB isproduced, the wavelength is 0.413 μm, the numerical aperture NA is 0.90,and the minimum pit length is 0.40 μm. Therefore, according to Formula(1), the proportionality constant K is 0.87.

On the other hand, in order to meet the demand for the optical dischaving the information capacity of 25 GB, the minimum pit length must bedecreased to 0.17 μm, and the track pitch must be decreased to about0.32 μm.

In general, shortening the wavelength of the laser is effective fornanofabrication of the irregular pattern (i.e., the formation ofsubmicron pits). As described above, in order to meet the demand for thehigh-density optical disc having the information capacity of 25 GB onthe single side, the minimum pit length must be decreased to about 0.17μm. In this case, if the proportionality constant K is 0.87 and thenumerical aperture NA is 0.95, the light source must include laserequipment having the wavelength λ of 0.18 μm.

ArF laser having a wavelength of 193 nm has been developing so that thelaser is used as a light source for semiconductor lithography for thenext-generation. The above wavelength, i.e., 0.18 μm, is shorter thanthe wavelength of the ArF laser. An exposure system that achieves anexposure with such a short wavelength is very expensive because theexposure system requires not only the special laser used as the lightsource, but also special optical parts such as a special lens.Accordingly, the above method for achieving nanofabrication, in whichthe wavelength λ during exposure is shortened and the numerical apertureNA of the objective lens is increased in order to increase the opticalresolution, is not extremely suitable for producing inexpensive devices.The reason is that, as the patterns become fine, the existing exposuresystems cannot be used and more expensive exposure systems must beintroduced instead. Accordingly, even if the performance of the laserequipment in an exposure system is improved, the increase of therecording capacity in the optical disc is limited.

In a general present exposing step, organic resists such as novolacresists and chemically amplified resists are exposed with ultravioletrays as the light source. The organic resists are all-purpose and widelyused in the photolithographic field. Unfortunately, the patterns on theboundaries between the exposed areas and the unexposed areas are notclear because of the high molecular weight of the organic resists.Accordingly, in terms of the precision, the organic resists cannot beused for the nanofabrication of the optical disc having a high capacitylevel of 25 GB.

On the other hand, inorganic resists, in particular, amorphous inorganicresists provide clear patterns on the boundaries between the exposedareas and the unexposed areas because the minimum structure unit of theinorganic resist is an atomic level. Therefore, the inorganic resistsare suitable for the precise nanofabrication compared with the organicresists. The use of the inorganic resists is promising to produce theoptical disc having a high capacity. For example, in a knownnanofabrication process, a resist material such as MoO₃ or WO₃ isexposed with ion beam as the light source (see, for example, NobuyoshiKoshida, Kazuyoshi Yoshida, Shinichi Watanuki, Masanori Komuro, andNobufumi Atoda: “50-nm Metal Line Fabrication by Focused Ion Beam andOxide Resists”, Jpn. J. Appl. Phys. Vol. 30 (1991) pp. 3246). In otherknown process, a resist material composed of SiO₂ is exposed withelectron beam as the light source (see, for example, Sucheta M.Gorwadkar, Toshimi Wada, Satoshi Hiraichi, Hiroshi Hiroshima, KenichiIshii, and Masanori Komuro: “SiO₂/c—Si Bilayer Electron-Beam ResistProcess for Nano-Fabrication”, Jpn. J. Appl. Phys. Vol. 35 (1996) pp.6673). Furthermore, a process has been studied in which a resistmaterial composed of chalcogenide glasses is exposed with laser havingthe wavelength of 476 nm and 532 nm, and a mercury xenon lamp thatradiates ultraviolet rays as the light source (see, for example, S. A.Kostyukevych: Investigations and modeling of physical processes ininorganic resists for the use in UV and laser lithography”, SPIE Vol.3424 (1998) pp. 20).

As described above, when ion beam or electron beam is used as the lightsource of the exposure, many kinds of inorganic resist material can beused in combination. In addition, the fine convergence of the electronbeam or the ion beam allows the irregular patterns to be minimized.However, an apparatus having the electron beam or the ion beam as theirradiation source has a complicated structure and is very expensive.Unfortunately, this apparatus is not suitable for producing aninexpensive optical disc.

In terms of the manufacturing cost, ultraviolet rays or visible light,that is, light from, for example, laser equipment installed in theexisting exposure system, is preferably used. However, a limitedmaterial of the inorganic resists can be patterned to form the cutportions using ultraviolet rays or visible light. Chalcogenide is theonly material that can be patterned using ultraviolet rays or visiblelight so far. The materials of the inorganic resists other thanchalcogenide transmit ultraviolet rays or visible light, and barelyabsorb the light energy. Accordingly, these materials are not suitablefor the practical use.

From an economical point of view, the use of the existing exposuresystem and chalcogenide is a practical combination. Unfortunately,chalcogenide includes materials that are harmful to the human body, forexample, Ag₂S₃, Ag—Ag₂S₃, and Ag₂Se—GeSe. Therefore, in terms of theindustrial production, the use of chalcogenide is difficult.

As described above, the optical disc having a high recording capacitycannot be manufactured with the existing exposure system so far.

In order to solve the above problems, it is an object of the presentinvention to provide a method for manufacturing an optical disc masterand a method for manufacturing an optical disc having a higher recordingcapacity. In the method for manufacturing an optical disc master,expensive irradiation equipment having, for example, electron beam orion beam is not used, instead, a safe resist material suitable forprecise nanofabrication and the existing exposure system are used.

SUMMARY OF THE INVENTION

As described above, completely oxidized transition metals (i.e.,complete oxides of transition metals) such as MoO₃ or WO₃ have been usedas resist materials for electron beam exposure or ion beam exposure.However, these oxides are clear to ultraviolet rays and visible light,and barely absorb the light. Accordingly, these oxides are not suitablefor the nanofabrication that employs ultraviolet rays or visible lightas the light source for exposure.

As a result of intensive study, the present inventors have found thefollowing phenomena: A slight shift of oxygen content from thestoichiometric composition of the transition metal oxides dramaticallyincreases the absorption of ultraviolet rays or visible light. Absorbingultraviolet rays or visible light changes chemical properties of thetransition metal oxides. Therefore, the metal oxides can be applied tothe resist material, and to a method for producing an optical discmaster. In other words, the proportionality constant K is improved inthe above Formula (1), thereby decreasing the minimum pit length P.

A method for manufacturing an optical disc master according to thepresent invention is based on the above fact. The method formanufacturing an optical disc master includes the steps of forming aresist layer composed of a resist material including an incompletelyoxidized transition metal (i.e., incomplete oxide of a transition metal)on a substrate, the oxygen content of the incomplete oxide being smallerthan the oxygen content of the stoichiometric composition correspondingto the valence of the transition metal; selectively exposing the resistlayer according to a recording signal pattern; and developing the resistlayer to form a predetermined irregular pattern.

According to a method for manufacturing an optical disc of the presentinvention, an optical disc master is used to produce the disc having anirregular pattern thereon, the master being produced by the steps offorming a resist layer composed of a resist material including anincomplete oxide of a transition metal on a substrate, the oxygencontent of the incomplete oxide being smaller than the oxygen content ofthe stoichiometric composition corresponding to the valence of thetransition metal; selectively exposing the resist layer according to therecording signal pattern; and developing the resist layer to form thepredetermined irregular pattern.

The above incomplete oxide of a transition metal is defined as acompound wherein the oxygen content of the oxide is shifted to besmaller than the oxygen content of the stoichiometric compositioncorresponding to a valence of the transition metal. In other words, theoxygen content of the incomplete oxide of a transition metal is smallerthan the oxygen content of the stoichiometric composition correspondingto the valence of the transition metal.

If the incomplete oxide includes a plurality of kinds of transitionmetals, one kind of transition metal atoms that have a crystal structureare partly replaced with other transition metal atoms. In this case, thedetermination of the incomplete oxides depends on the fact if the oxygencontent of the oxide is smaller than the oxygen content of thestoichiometric compositions of the plurality of kinds of the transitionmetal.

According to the present invention, since the incomplete oxide of atransition metal used as the resist material absorbs ultraviolet rays orvisible light, the resist can be exposed without using a special lightsource for exposure, such as electron beam or ion beam. Furthermore,since the incomplete oxide of a transition metal has a low molecularweight, the boundaries between the unexposed areas and the exposed areaare clear compared with an organic resist having a high molecularweight. Accordingly, the use of the incomplete oxide of a transitionmetal provides a highly precise resist pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 includes drawings illustrating a manufacturing process of anoptical disc according to a method for manufacturing the optical disc ofthe present invention.

FIG. 2 is a schematic view of an exposure system used in a method formanufacturing an optical disc master according to the present invention.

FIG. 3 is a characteristic graph showing a relationship of anirradiation power of the light source used for the exposure and adifference of the etching rate between an exposed area and an unexposedarea in the case where a resist layer composed of a resist materialaccording to the present invention is exposed.

FIGS. 4A to 4C are characteristic graphs showing an example of anirradiation pattern in the exposing step. FIGS. 4A and 4B show examplesof irradiation pulses, and FIG. 4C shows an example of continuous light.

FIGS. 5A to 5D are schematic sectional views of the principal partsillustrating a bilayer resist process. FIG. 5A illustrates a step offorming a first resist layer and a second resist layer, FIG. 5Billustrates a step of patterning the first resist layer, FIG. 5Cillustrates a step of etching the second resist layer, and FIG. 5Dillustrates a step of removing the first resist layer.

FIG. 6 is an SEM image of a resist layer composed of an incomplete oxideof tungsten (W) after the development.

FIG. 7 is an SEM image of a resist layer composed of an incomplete oxideof tungsten (W) and molybdenum (Mo) after the development.

FIG. 8 is an SEM image of a pit pattern formed on the surface of anoptical disc having a recording capacity of 25 GB that was produced inExample 2.

FIGS. 9A to 9C show evaluation results of signals in the optical dischaving the recording capacity of 25 GB that was produced in Example 2.

FIG. 10 includes drawings illustrating a known manufacturing process ofan optical disc.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

An embodiment of a method for manufacturing an optical disc according tothe present invention will now be described in detail with reference tothe drawings.

The summary of a manufacturing process according to the method formanufacturing an optical disc of the present invention will now bedescribed with reference to FIG. 1.

A resist layer 102 composed of a predetermined inorganic resist materialis uniformly deposited on a substrate 100 by sputtering (a step offorming a resist layer, FIG. 1( a)). The material of the resist layer102 will be described later in detail. A predetermined interlayer 101may be formed between the substrate 100 and the resist layer 102 toimprove the exposure sensitivity of the resist layer 102. FIG. 1( a)illustrates this case. Although the resist layer 102 may have anythickness, the resist layer 102 preferably has a thickness of 10 to 80nm.

Subsequently, the resist layer 102 is selectively exposed according to asignal pattern with an exposure system having existing laser equipment(a step of exposing the resist layer FIG. 1( b)). The resist layer 102is developed to prepare a master 103 having a predetermined irregularpattern thereon (a step of developing the resist layer FIG. 1( c)).

Then, a metallic nickel film is formed on the irregular pattern of themaster 103 by electroforming (FIG. 1( d)). The nickel film is lifted offfrom the master 103, and then a predetermined process is performed toproduce a molding stamper 104 having the irregular pattern of the master103 (FIG. 1( e)).

Polycarbonate, which is a thermoplastic resin, is molded by injectionmolding using the molding stamper 104 to form a resin disc substrate 105(FIG. 1( f)). Subsequently, the stamper is removed (FIG. 1( g)), areflecting film 106 composed of, for example, an aluminum alloy (FIG. 1(h)) and a protective film 107 having a thickness of about 0.1 mm areformed on the irregular surface of the resin disc substrate 105 toproduce an optical disc (FIG. 1( i)). The steps of manufacturing theoptical disc using the resist master (i.e., the master having the resistthereon) may be performed with a known art.

[Resist Materials]

The resist material used for the resist layer 102 is composed ofincomplete oxide of a transition metal. The incomplete oxide of atransition metal is defined as a compound wherein the oxygen content ofthe oxide is shifted to be smaller than the oxygen content of thestoichiometric composition corresponding to a valence of the transitionmetal. In other words, the oxygen content of the incomplete oxide of atransition metal is smaller than the oxygen content of thestoichiometric composition corresponding to the valence of thetransition metal.

A chemical formula MoO₃ will now be described as an example of theincomplete oxide of a transition metal. The oxidation state of thechemical formula MoO₃ is converted into a composition ratio ofMo_(1-x)O_(x). When the value x is 0.75 (i.e., x=0.75), the compound isa complete oxide. When the value x is represented by 0<x<0.75, thecompound is an incomplete oxide in which the oxygen content of thecompound is smaller than the oxygen content of the stoichiometriccomposition.

Some transition metals can form its oxides that have different valences.In this case, if the actual oxygen content of an oxide is smaller thanthe oxygen content of the stoichiometric composition corresponding tothe valence of the transition metal, the compound is defined as anincomplete oxide according to the present invention. For example, theoxides of molybdenum (Mo) include not only the above trivalent oxide(MoO₃), which is the most stable compound, but also a monovalent oxide(MoO). In this case, the oxidation state is converted into a compositionratio of Mo_(1-x)O_(x). When the value x is represented by 0<x<0.5, thecompound is an incomplete oxide in which the oxygen content of thecompound is smaller than the oxygen content of the stoichiometriccomposition. The valence of the transition metal oxide can be determinedwith a commercially available analytical instrument.

The incomplete oxide of the transition metal absorbs ultraviolet rays orvisible light. Irradiating ultraviolet rays or visible light changeschemical properties of the incomplete oxide of the transition metal.Consequently, as described later in detail, in spite of an inorganicresist, the exposed areas and the unexposed areas of the resist havedifferent etching rates in the developing step. That is, the resist hasselectivity. Furthermore, according to the resist material composed ofthe incomplete oxide of the transition metal, since the size of themicroparticle of the resist film material is small, the pattern on theboundaries between the exposed areas and the unexposed areas becomeclear. Accordingly, the resolution can be improved.

Since the property of the resist material composed of the incompleteoxide of the transition metal depends on the degree of the oxidation,the optimum degree of the oxidation must be appropriately selected. Ifthe oxygen content of the incomplete oxide of the transition metal isconsiderably smaller than that of the stoichiometric composition of thecomplete oxide, some disadvantages arise. For example, the exposing steprequires a high irradiation power and the developing step takes a longtime. Preferably, the oxygen content of the incomplete oxide of thetransition metal is slightly smaller than that of the stoichiometriccomposition of the complete oxide.

Examples of the transition metal used as the resist material include Ti,V, Cr, Mn, Fe, Nb, Cu, Ni, Co, Mo, Ta, W, Zr, Ru, and Ag. Preferably,Mo, W, Cr, Fe, and Nb are used. More preferably, Mo and W are used interms of considerable chemical change by irradiation of ultraviolet raysor visible light.

According to the present invention, the incomplete oxide of thetransition metal may be an incomplete oxide of a first transition metal.The incomplete oxide of the first transition metal may further include asecond transition metal. The incomplete oxide of the first transitionmetal may further include a plurality of kinds of transition metals. Theincomplete oxide of the first transition metal may further include atleast one element other than transition metals. According to the presentinvention, in particular, the incomplete oxide of the transition metalpreferably includes a plurality of kinds of metal elements.

If the incomplete oxide of the first transition metal further includes asecond transition metal, or further includes at least three transitionmetals, the first transition metal atoms that have a crystal structureare partly replaced with other transition metal atoms. In this case, thedetermination of the incomplete oxides depends on the fact if the oxygencontent of the oxide is smaller than the oxygen content of thestoichiometric compositions of the plurality of kinds of the transitionmetals.

Examples of the element other than transition metal include at least oneelement selected from the group consisting of, for example, Al, C, B,Si, and Ge. The use of at least two kinds of transition metals incombination or adding at least one element other than transition metaldecreases the crystal grain size of the incomplete oxides of thetransition metal. Accordingly, the boundaries between the exposed areasand unexposed areas become clearer, thereby significantly improving theresolution. The exposure sensitivity can be also improved.

The above resist material can be produced in an atmosphere includingargon and oxygen by sputtering using a target containing thepredetermined transition metal. For example, the content of the oxygenis 5% to 20% of the total gas flow introduced in a chamber at a normalsputtering gas pressure (1 to 10 Pa). [Method for Producing Optical DiscMaster]

A method for manufacturing an optical disc master, the method being abackbone of the method for manufacturing the optical disc, will now bedescribed in detail.

An embodiment of the method for manufacturing the optical disc masteraccording to the present invention, for example, includes the steps offorming a resist material composed of an incomplete oxide of atransition metal on a substrate to form a resist layer; selectivelyexposing the resist layer; and developing the resist layer to produce amaster having a predetermined irregular pattern thereon, as describedabove. Each step will now be described in detail.

[Step of Forming Resist Layer]

A resist layer composed of an incomplete oxide of a transition metal isformed on a substrate having a sufficiently smooth surface. Examples ofthe method include a deposition by sputtering in an atmosphere includingargon and oxygen with a sputtering target composed of the transitionmetal. In this case, changing the content of oxygen gas in vacuum cancontrol the degree of oxidation of the incomplete oxide of thetransition metal. When incomplete oxides of transition metals includingat least two kinds of transition metals are deposited by sputtering, thesubstrate is constantly rotated on the different kinds of sputteringtargets to mix the plurality of kinds of transition metals. The mixingratio of the transition metals is controlled by individually changingthe sputtering power.

In order to deposit the resist layer composed of the incomplete oxide ofthe transition metal, as described above, the sputtering may beperformed in an atmosphere containing oxygen with the metal target.Alternatively, the sputtering may be performed in argon atmosphere asusual with a target composed of the incomplete oxide of the transitionmetal that has the predetermined oxygen content.

Furthermore, in addition to sputtering, the resist layer composed of theincomplete oxide of the transition metal can be readily deposited byvapor deposition.

Examples of the substrate include glass; a plastic such aspolycarbonate; silicon; alumina-titanium carbide; and nickel.

Although the resist layer may have any thickness, the resist layer mayhave a thickness of, for example, 10 to 80 nm.

[Step of Exposing Resist Layer]

The substrate after deposition of the resist layer (hereinafter referredto as resist substrate 1) is disposed on a turntable 11 of an exposuresystem shown in FIG. 2 such that the face having the resist layerthereon is placed on the upper side.

The exposure system includes a beam source 12 that emits light, such aslaser, to expose the resist layer. The beam source 12 irradiates theresist layer of the resist substrate 1 with laser. The laser is focusedon the resist layer through a collimator lens 13, a beam splitter 14,and an objective lens 15. According to this exposure system, reflectedlight from the resist substrate 1 is converged on a split photo detector17 through the beam splitter 14 and a converging lens 16. The splitphoto detector 17 detects the reflected light from the resist substrate1, generates a focus error signal 18 based on the detection result, andsends the focus error signal 18 to a focus actuator 19. The focusactuator 19 controls the position of the objective lens 15 in thevertical direction. The turntable 11 includes a feeding attachment (notshown in the figure) to change the exposing position of the resistsubstrate 1 precisely. According to this exposure system, the exposureor the focusing is performed while a laser driving circuit 23 iscontrolling the beam source 12 based on a data signal 20, a reflectedlight intensity signal 21, and a tracking time difference signal 22.Furthermore, a spindle motor control system 24 is disposed at thecentral axis of the turntable 11. The spindle motor control system 24determines an optimal revolution speed of a spindle based on the radialposition of the optical system and a desired linear velocity, thuscontrolling a spindle motor.

In an exposing step where a known organic resist is used as a resistlayer, the focusing to the resist layer is not performed with theexposing light source itself . The reason is as follows: The chemicalproperty of the organic resist is continuously changed by exposure.Therefore, even though light for focusing is faint, the resist layercomposed of the organic resist is unnecessarily exposed by theirradiation. Accordingly, additional light source that emits lighthaving a wavelength to which the organic resist is not sensitive, forexample, a red light source that emits light having a wavelength of 633nm, is prepared to perform the focusing. As described above, since theexposure system used for the known organic resist uses two light sourcesthat emit light having different wavelength, the exposure systemrequires an optical system that can perform wavelength division.Unfortunately, the exposure system requires a very complicated opticalsystem and the cost of the exposure system is increased. Furthermore, inthe exposure system used for the known organic resist, the resolution bythe focus error signal, which is used for controlling the position ofthe objective lens in the vertical direction, is proportional to thewavelength of the light source (for example, wavelength: 633 nm) usedfor the detection. Accordingly, the resolution is not as high as aresolution accomplished by the light source used for the exposure.Unfortunately, a precise and stable focusing cannot be performed.

On the other hand, according to the inorganic resist material of thepresent invention, the chemical property of the resist changes veryrapidly in the exposure. FIG. 3 shows the relationship of an irradiationpower of a light source used for the exposure and a difference of theetching rate (i.e., contrast) between an exposed area and an unexposedarea. When the irradiation power is less than an irradiation thresholdpower PO at which the exposure starts, even repeated irradiation doesnot cause unnecessary exposure. Accordingly, the focusing can beperformed with the exposing light source itself at an irradiation powersmaller than the P0. According to the method for manufacturing theoptical disc master of the present invention, the exposure system doesnot require an optical system that performs wavelength division, therebydecreasing the cost of the exposure system. Furthermore, since a highlyprecise focusing that corresponds to the wavelength in exposure can beachieved, an accurate nanofabrication can be performed. The resistmaterial of the present invention, which is an inorganic resist, is notsensitized with faint light having an irradiation power smaller than theirradiation threshold power P0. Therefore, unlike the process in whichthe known organic resist is used, cutting ultraviolet light in roomlighting is not necessary.

As described above, the focusing is performed with light having theirradiation power smaller than the irradiation threshold power P0, andthe turntable 11 is then moved at a desired radial position. In thiscase, the optical system including such as the objective lens 15 isfixed in position in the longitudinal direction, whereas the turntable11 is moved in order to change the exposure position of the resistsubstrate 1. Alternatively, of course, the turntable 11 having theresist substrate 1 thereon may be fixed, whereas the position of theoptical system may be changed.

Subsequently, the beam source 12 radiates laser on the resist layer, andthe turntable 11 is rotated at the same time in order to expose theresist layer. In this exposing step, in order to form a fine irregularlatent image, the turntable 11 is continuously moved in the radialdirection of the resist substrate 1 by a small pitch, while theturntable 11 is kept rotating. For example, in order to produce arecording disc, a spiral pregroove is formed as the fine irregularlatent image. In order to produce an optical disc, irregular pitsrepresenting information data, and a meandering pregroove are formed asthe fine irregular latent image. In order to produce a disc that hasconcentric tracks, for example, a magnetic hard disc, the turntable 11or the optical system is moved not continuously but stepwise.

According to the conditions described above, irradiation pulses orcontinuous light having a desired power, which is larger than or equalto the irradiation threshold power P0, is irradiated in order on theresist layer from a desired position of the resist substrate 1corresponding to the pits or the pregroove based on the informationdata. Thus, the exposure is performed. FIGS. 4A and 4B show examples ofirradiation pulses, and FIG. 4C shows an example of continuous light.

According to the resist material of the present invention composed of anincomplete oxide of a transition metal, the chemical property of theresist is changed by irradiation of ultraviolet rays or visible lighthaving a power larger than or equal to the irradiation threshold powerP0. Consequently, the exposed areas and the unexposed areas of theresist have different etching rates in an alkali or an acid. That is,the resist has selectivity.

A low irradiation power can form a short and narrow pit. However, anexcessively low irradiation power gets close to the irradiationthreshold power P0, and therefore, prevents the stable patternformation. Accordingly, the exposure must be appropriately performedwith an optimal irradiation power.

The present inventors have verified that the exposure using a redsemiconductor laser that emits light having a wavelength of 660 nm and amercury lamp that emits light having peaks at wavelengths of 185 nm, 254nm, and 405 nm provides the resist material of the present inventionwith the selectivity, and this process can form a fine pit pattern.

[A Step of Developing Resist Layer]

Subsequently, the resist substrate 1 having the exposed pattern asdescribed above is developed to produce a resist master used forproducing an optical disc. The resist master has a fine irregularsurface including the pits or the pregroove corresponding to thepredetermined exposure pattern.

The step of developing includes a wet process using, for example, anacidic solution or an alkaline solution. This process provides theresist layer with selectivity. The step of developing may beappropriately changed depending on, for example, the intended use, theapplication, and the device and equipment. Examples of the alkalinedeveloper include a solution of tetramethylammonium hydroxide; andsolutions of inorganic alkali such as KOH, NaOH, and Na₂CO₃. Examples ofthe acidic developer include hydrochloric acid, nitric acid, sulfuricacid, and phosphoric acid. The present inventors have verified that inaddition to the wet process, a dry process such as plasma etching, i.e.,reactive ion etching (RIE) can be also used for the development in whichthe kinds of the gas and the mixing ratio of a plurality of gases arecontrolled.

A method for controlling the exposure sensitivity will now be described.Take an example where the oxidation state of a transition metal oxiderepresented by chemical formula WO₃ is converted into a compositionratio of W_(1-x)O_(x). When the value x is represented by 0.1<x<0.75, ahigh exposure sensitivity can be achieved. When the value x is 0.1, thisvalue is a critical value in which, for example, the exposing steprequires a high irradiation power and the developing process takes along time disadvantageously. When the value x is in the range of about0.4 to about 0.7, the highest exposure sensitivity can be achieved.

Take an example where the oxidation state of a transition metal oxiderepresented by chemical formula MoO₃ is converted into a compositionratio of Mo_(1-x)O_(x). When the value x is represented by 0.1<x<0.75, ahigh exposure sensitivity can be achieved. When the value x is 0.1, thisvalue is a critical value in which, for example, the exposing steprequires a high irradiation power and the developing process takes along time disadvantageously. When the value x is in the range of about0.4 to about 0.7, the highest exposure sensitivity can be achieved.

Furthermore, take an example where the oxidation state of a transitionmetal oxide represented by chemical formula MoO is converted into acomposition ratio of Mo_(1-x)O_(x). When the value x is represented by0.1<x<0.5, a high exposure sensitivity can be achieved. When the value xis 0.1, this value is a critical value in which, for example, theexposing step requires a high irradiation power and the developingprocess takes a long time disadvantageously.

A high exposure sensitivity of the resist material, for example,advantageously decreases the irradiation power during exposure anddecreases the exposing time corresponding to the pulse width and thelinear velocity. However, excessively high exposure sensitivitydisadvantageously causes unnecessary exposure during focusing, andcauses an adverse effect in the exposure due to the lighting environmentin the processing room. Accordingly, optimal exposure sensitivity isappropriately selected depending on the application. In order to controlthe exposure sensitivity of the resist material according to the presentinvention, the oxygen content of the material is increased or decreased;alternatively, a second transition metal is effectively added to anincomplete oxide of a first transition metal. For example, addingmolybdenum (Mo) to W_(1x)O_(x) can improve the exposure sensitivity byabout 30%.

Furthermore, in addition to the change of the resist materialcomposition, the selection of the substrate material and pretreatmentsfor exposure on the substrate can also control the exposure sensitivity.The dependency of the substrate material to the exposure sensitivity wasinvestigated using quartz, silicon, glass, and a plastic(polycarbonate). As a result, the exposure sensitivity depended on thesubstrate material, more specifically; the highest exposure sensitivitywas achieved with the plastic, subsequently, glass, quartz, and silicon,in this order. This order corresponds to the order of the thermalconductivity. A substrate having a small thermal conductivity achieveshigh exposure sensitivity. The reason is as follows: The use of thesubstrate having a small thermal conductivity significantly increasesthe temperature of the resist material during exposure. Subsequently, asthe temperature increases, the chemical property of the resist materialis significantly changed.

Examples of the pretreatments for exposure include a formation of aninterlayer disposed between the substrate and the resist material, aheat treatment, and ultraviolet irradiation.

In particular, when a substrate having a large thermal conductivity, forexample, silicon wafer composed of single crystal silicon is used, aninterlayer having a relatively small thermal conductivity is preferablyformed on the substrate to appropriately improve the exposuresensitivity. The reason is that the interlayer enhances the thermalstorage in the resist material during exposure. Examples of the materialof the interlayer having a small thermal conductivity include amorphoussilicon, silicon dioxide (SiO₂), silicon nitride (SiN), and alumina(Al₂O₃). The interlayer may be formed by sputtering or other vacuumdepositions.

UV curable resin layer having a thickness of 5 μm was formed on a quartzsubstrate by spin coating. Ultraviolet rays were then irradiated to curethe liquid resin. The exposure sensitivity in the above substrate washigher than that in the untreated quartz substrate. This is also becausethe thermal conductivity of the UV curable resin is as low as a plastic.

Other pretreatments for exposure, for example, a heat treatment andultraviolet irradiation can also improve the exposure sensitivity.Although the effect is not perfect, these pretreatments allow thechemical property of the resist material of the present invention to bechanged at some level.

As described above, appropriate choices of the resist materialcomposition, the developing condition, and the substrate material canexpress functions of the resist composed of an incomplete oxide of atransition metal, and having various properties. Furthermore, in orderto expand the application of the resist material, a bilayer resistprocess (i.e., a process using bilayer resist) is very useful. Theoutline of the bilayer resist process will now be described withreference to FIGS. 5A to 5D.

Referring to FIG. 5A, a first resist layer 30 is composed of anincomplete oxide of a transition metal according to the presentinvention. Before the deposition of the first resist layer 30, a secondresist layer 32 is deposited on a substrate 31. The selectivity of thematerial of the second resist layer 32 and the selectivity of theincomplete metal oxide of the transition metal in the first resist layer30 are significantly different.

Subsequently, as shown in FIG. 5B, the first resist layer 30 is exposedand is then developed to form a pattern thereon.

Then, the second resist layer 32 is etched under a high selectiveetching condition by using the pattern of the first resist layer 30 as amask. As shown in FIG. 5C, the pattern of the first resist layer 30 iscopied on the second resist layer 32.

Finally, the first resist layer 30 is removed. Thus, as shown in FIG.5D, the patterning of the second resist layer 32 is completed.

In order to apply the present invention to the bilayer resist process,for example, the substrate is composed of quartz, the second resistlayer is composed of a transition metal such as Cr, and the etching isperformed by RIE, i.e., plasma etching with a chlorofluorocarbon gas. Inthis case, the difference of the selectivity between the incompleteoxide of the transition metal in the first resist layer 30 and thesecond resist layer 32 becomes approximately the largest.

As described above, the above resist material composed of an incompleteoxide of a transition metal is used in the method for manufacturing anoptical disc master of the present invention. Accordingly, even thoughthe resist is composed of an inorganic material, the resist can beadvantageously exposed with ultraviolet rays or visible light. Thisproperty is absolutely different from that of the known inorganicresist: Since the known inorganic resist is optically clear toultraviolet rays or visible light, the ultraviolet rays or the visiblelight cannot be used as the light source for exposure. Subsequently, anexpensive exposure system that uses, for example, electron beam or ionbeam is essential to exposure the known inorganic resist.

Since the use of the ultraviolet rays or visible light achieves a highimaging speed, the time required for the exposure can be significantlydecreased, compared with a known method for producing an optical discmaster in which the electron beam is used as the light source in orderto expose the known inorganic resist.

The use of the inorganic resist material composed of an incomplete oxideof a transition metal provides clear patterns at the boundaries betweenthe exposed areas and the unexposed areas, thus achieving a highlyprecise nanofabrication. Furthermore, since the focusing can beperformed with the exposing light source itself in the exposing step, ahigh resolution can be achieved.

As described above, according to the method for manufacturing an opticaldisc master of the present invention, the proportionality constant K inthe formula P=K·λ/NA is decreased in order to achieve thenanofabrication. Unlike the known method in which the wavelength λ inexposure is shortened and the numerical aperture NA of the objectivelens is increased to achieve the nanofabrication, the method of thepresent invention can perform more precise patterning with the existingexposure system. Specifically, according to the present invention, theproportionality constant K can be decreased to less than 0.8, and aminimum fine patterning cycle f of a workpiece can be decreased asfollows:

f<0.8λ/NA

According to the present invention, the existing exposure system can beused without further improvement. Therefore, the present inventioninexpensively provides an optical disc master on which a more precisenanofabrication is performed.

EXAMPLES

Examples according to the present invention will now be described withreference to the experimental results.

EXAMPLE 1

In Example 1, a master having a resist thereon (i.e., resist master)used for producing an optical disc was actually produced using a resistmaterial composed of an incomplete oxide of trivalent tungsten (W).

A resist layer composed of an incomplete oxide of tungsten was uniformlydeposited by sputtering on a glass substrate having a sufficientlysmooth surface. The sputtering was performed with a sputtering targetcomposed of tungsten element in an atmosphere containing argon andoxygen. The content of oxygen gas was changed in order to control thedegree of oxidation of the incomplete oxide of tungsten.

The composition of the deposited resist layer was analyzed with anenergy dispersive X-ray spectrometer (EDX) . When the composition ratiowas represented by W_(1-x)O_(x), the value x was 0.63. The thickness ofthe resist layer was controlled to be 40 nm. The wavelength dependenceof the refractive index was measured by spectroscopic ellipsometry.

The substrate having the deposited resist layer thereon, i.e., resistsubstrate, was disposed on the turntable of the exposure system shown inFIG. 2. While the turntable was rotated at a desired revolution speed,laser having less than the irradiation threshold power was irradiated onthe resist layer. The actuator controlled the position of the objectivelens in the vertical direction to focus on the resist layer.

Subsequently, the turntable was moved at a desired radial position withthe feeding attachment attached to the turntable, whereas the opticalsystem was fixed. Based on the information data, irradiation pulsescorresponding to the pits were irradiated on the resist layer to exposethe resist layer. In the exposing step, the turntable was continuouslymoved in the radial direction of the resist substrate by a small pitch,while the turntable was kept rotating. The wavelength in the exposurewas 405 nm and the numerical aperture NA of the exposing optical systemwas 0.95. The linear velocity in the exposing step was 2.5 m/s and theirradiation power was 6.0 mW.

After the exposure, the resist substrate was developed by a wet processwith an alkaline developer. In this developing step, the resistsubstrate was submerged in the developer and ultrasonic waves wereapplied in order to etch the resist uniformly. After the development,the substrate was sufficiently washed with purified water and isopropylalcohol and was dried by, for example, blowing air to finish theprocess. The alkaline developer was a solution of tetramethylammoniumhydroxide and the developing time was 30 minutes.

FIG. 6 is a scanning electron microscope (SEM) image of the resistpattern after the development. Referring to FIG. 6, the pits correspondwith the exposed areas. The exposed areas formed hollows relative to theunexposed area on the resist layer. That is, the resist materialcomposed of the incomplete oxide of tungsten was a positive resist. Inthe resist layer composed of the incomplete oxide of tungsten, theetching rate of the unexposed area was smaller than that of the exposedareas. Accordingly, the unexposed areas of the resist layer had athickness almost the same as the thickness after the deposition. On theother hand, the exposed areas of the resist layer were removed byetching. Consequently, the surface of the glass substrate was exposed atthe exposed areas.

The smallest pit of the four pits shown in FIG. 6 has the width of 0.15μm and the length of 0.16 μm. Accordingly, the method for manufacturingthe optical disc master wherein the resist material of the presentinvention is used, can significantly improve the resolution relative tothe known method with an organic resist, in which the expected pit widthis 0.39 μm. Furthermore, FIG. 6 shows that the pit has a very clearedge.

The experimental results also showed that the width and the length ofthe pit formed after developing depended on the irradiation power andthe pulse width of the light source for exposure.

Comparative Example 1

In Comparative Example 1, a resist master used for producing an opticaldisc was actually produced using a resist material composed of acomplete oxide of tungsten, i.e., WO₃.

A resist layer composed of the complete oxide of tungsten was depositedby sputtering on a glass substrate. According to the analytical resultby the EDX, when the composition ratio of the deposited resist layer wasrepresented by W_(1-x)O_(x), the value x was 0.75. By the way, theanalytical result of electron diffraction by a transmission electronmicroscope showed that before the exposure, the crystal state of theincomplete tungsten oxide (WO) was amorphous.

This resist layer was exposed with the same irradiation power as inExample 1 or a sufficiently strong irradiation power. However, theselectivity in the resist layer was 1 or less, and the desired pitpattern was not formed. Since the complete oxide of tungsten wasoptically clear to the light source for exposure, the complete oxide oftungsten barely absorbed the light. The small absorption could notchemically change the resist material.

Example 2

In Example 2, a resist master used for producing an optical disc wasactually produced using a resist material composed of an incompleteoxide of trivalent tungsten and trivalent molybdenum according to themanufacturing process shown in FIG. 1. Then, the optical disc wasfinally manufactured. The operating process will now be described withreference to FIG. 1.

Firstly, an interlayer 101 composed of amorphous silicon and having athickness of 80 nm was uniformly deposited on a substrate 100 that is asilicon wafer by sputtering. Subsequently, a resist layer 102 composedof an incomplete oxide of tungsten (W) and molybdenum (Mo) was uniformlydeposited on the substrate by sputtering (FIG. 1( a)). The sputteringwas performed in argon atmosphere with a sputtering target composed ofthe incomplete oxide of tungsten and molybdenum. According to theanalytical result of the deposited resist by the EDX, the ratio of thetungsten and molybdenum in the deposited incomplete oxide of tungstenand molybdenum was 80:20, and the oxygen content of the incomplete oxidewas 60 atomic percent. The resist layer had the thickness of 55 nm. Theanalytical result of electron diffraction by the transmission electronmicroscope showed that before the exposure, the crystal state of theabove incomplete oxide (WMoO) was amorphous.

Regarding the step of exposing of the resist layer and the subsequentsteps, all conditions except for the exposing condition were the same asin Example 1. Thus, a resist master 103 used for producing the opticaldisc was produced (FIG. 1( b) and FIG. 1( c)). The exposing condition inExample 2 was as follows:

-   -   Wavelength in exposure: 405 nm    -   Numerical aperture NA of exposing optical system: 0.95    -   Modulation: 17PP    -   Bit length: 112 nm    -   Track pitch: 320 nm    -   Linear velocity in exposing step: 4.92 m/s    -   Irradiation power in exposure: 6.0 mW    -   Writing system: Simple writing system the same as phase-change        disc

FIG. 7 is an SEM image showing an example of the resist pattern afterthe development of the resist master used for producing the opticaldisc. The resist material composed of the incomplete oxide of tungstenand molybdenum was a positive resist. Referring to FIG. 7, the pitscorrespond with the exposed areas. The exposed areas formed hollowsrelative to the unexposed area on the resist layer. The pit length(diameter) was about 130 nm. In other words, this pit length (diameter)was smaller than 170 nm (0.17 μm), which was required for the minimumpit length in the high density optical disc having the recordingcapacity of 25 GB on the single side. Furthermore, the resist patternincluded identically shaped pits with a constant pitch of 300 nm in thepit line direction and with a constant pitch of 320 nm in the trackdirection. This result showed that the pits were stably formed in thisExample.

Then, a metallic nickel film was formed on the irregular pattern of theresist master by electroforming (FIG. 1( d)). The nickel film was liftedoff from the resist master. Subsequently, a predetermined process wasperformed to produce a molding stamper 104 having the irregular patternof the resist master (FIG. 1( e)).

Polycarbonate, which was a thermoplastic resin, was molded by injectionmolding using the molding stamper to form a resin disc substrate 105(FIG. 1( f)). The stamper was removed (FIG. 1( g)), and then areflecting film 106 composed of an aluminum alloy (FIG. 1( h)) and aprotective film 107 having a thickness of 0.1 mm were formed on theirregular surface of the resin disc substrate to produce an optical dischaving a diameter of 12 cm (FIG. 1( i)). The above steps ofmanufacturing the optical disc using the resist master were performedaccording to the known art.

FIG. 8 is an SEM image showing an example of a pit pattern formed on thesurface of the above optical disc. Referring to FIG. 8, pits that wereformed on the optical disc corresponded to an actual signal patternincluding, for example, pits having the length of 150 nm and linear pitshaving the width of 130 nm. This result showed that the optical disc hadthe recording capacity of 25 GB.

Subsequently, the optical disc was read out under the followingcondition. The RF signals were converted into eye patterns to evaluatethe signals. FIGS. 9A to 9C show the results of the signal evaluation.

-   -   Tracking servo: Push-pull method    -   Modulation: 17PP    -   Bit length: 112 nm    -   Track pitch: 320 nm    -   Linear velocity in readout: 4.92 m/s    -   Irradiation power in readout: 0.4 mW

The jitter value of an eye pattern (FIG. 9B) generated by performingconventional equalization on an untreated readout eye pattern (FIG. 9A)was 8.0%. The jitter value of an eye pattern (FIG. 9C) generated byperforming limit equalization on the untreated readout eye pattern (FIG.9A) was 4.6%. These jitter values were sufficiently small for practicaluse of the optical disc as a ROM disc having the recording capacity of25 GB.

The photolithographic technology according to the present inventionincluding the steps from the formation of the resist layer to thedevelopment may be applied to produce various devices such assemiconductor devices, e.g. a dynamic random access memory (DRAM), aflash memory, a central processing unit (CPU), and an applicationspecific integrated circuit (ASIC); magnetic devices, e.g. magnetichead; display devices, e.g. a liquid crystal device, anelectroluminescence (EL) device, and a plasma display panel (PDP); andoptical devices, e.g. an optical recording medium and a light modulationdevice.

As described above, according to the method for producing an opticaldisc master of the present invention, the resist layer is composed of anincomplete oxide of a transition metal that absorbs ultraviolet rays orvisible light. Accordingly, an existing exposure system having anexposing light source that emits ultraviolet rays or visible light canbe used in order to expose the resist layer. Furthermore, since theresist material composed of the incomplete oxide of the transition metalhas a small molecular size, the developed resist layer has a superioredge pattern, thus achieving a highly precise patterning.

According to the method for manufacturing an optical disc using theoptical disc master described above, an optical disc having therecording capacity of 25 GB class can be produced with the existingexposure system.

1. An apparatus for manufacturing an optical disc master, comprising: aturntable upon which is received a disc having a resist layer composedof a resist material, the resist material comprising an incomplete oxideof a transition metal on a substrate, the oxygen content of theincomplete oxide being smaller than the oxygen content of thestoichiometric composition corresponding to a valence of the transitionmetal, which increases the absorption of ultraviolet rays and visiblelight rays, the resist material being an amorphous inorganic materialcontaining an oxide; and an exposure system operatively configured toselectively expose the resist layer to ultraviolet rays or visiblelight.
 2. The apparatus of claim 1, wherein the exposure system isoperatively configured to expose the resist layer to a light beam havingan irradiation power that is less than an irradiation threshold power atwhich exposure of the resist starts.
 3. An apparatus for manufacturingan optical disc master, comprising: a turntable upon which is received adisc having a resist layer composed of a resist material, the resistmaterial comprising an incomplete oxide of a transition metal on asubstrate, the oxygen content of the incomplete oxide being smaller thanthe oxygen content of the stoichiometric composition corresponding to avalence of the transition metal, which increases the absorption ofultraviolet rays and visible light rays, the resist material being anamorphous inorganic material containing an oxide; a light source; alight source control unit operatively configured to control the lightsource, and a lens system which configures light from the light sourceas a light beam which is directed onto the disc; wherein, the apparatusselectively exposes the resist layer to ultraviolet rays or visiblelight.
 4. The apparatus of claim 3, wherein the exposure system isoperatively configured to expose the resist layer to a light beam havingan irradiation power that is less than an irradiation threshold power atwhich exposure of the resist starts.
 5. The apparatus of claim 3,further comprising: a collimator lens through which light from the lightsource travels; a beam splitter operatively positioned and configured tosplit light from the collimator lens into a first beam and a secondbeam; an objective lens operatively configured and positioned to focusthe first beam onto the disc; a split photo detector operativelypositioned and configured to receive via the light from beam splitterlight which has reflected from the disc; a converging lens between thebeam splitter and the split photo detector; and a focus actuatoroperatively configured to adjust the position of the objective lensrelative to the turntable, wherein, the split photo detector isconfigured to generate a focus error signal based on a comparison of thesecond beam and the reflected light, and the focus actuator isconfigured to adjust the position of the objective lens in accordancewith the focus error signal.
 6. The apparatus of claim 3, wherein, thelight source is a laser.